This invention relates to novel adhesive compositions and to the easy removal of adhesive residue from a substrate without use of an organic solvent. More particularly, it relates to compositions which are especially suited for use as pressure sensitive adhesives characterized by their excellent properties of outstanding creep resistance and cohesive strength and which are based on polyvinyl methyl ether. These adhesive compositions can be in the form of hot-melts or solvent-based compositions, either water-based or organic solvent-based. Water based compositions are preferred. Adhesive residues are water-removable without use of an organic solvent.
Hot-melt adhesives are frequently used because they can be cheaper than organic solvent based formulations, can give quick tack in the molten state, harden quickly on cooling, have no residual solvent odor or taste and their application is often a one-step procedure. However, hot-melt adhesives are not normally applicable to substrates subject to degradation at hot-melt application temperatures. Organic solvent-based adhesives can be more expensive because of organic solvent requirement and the potential need for solvent recovery. Water-based adhesives can be preferred because of cost and reduced environmental problems.
Water-based adhesives are taught in the prior art. Plitt, U.S. Pat. No. 2,985,609, teaches a water-based pressure sensitive adhesive comprising a composition of polyvinyl alcohol and polyethyleneimine. However, as Davis, U.S. Pat. No. 3,249,572 reports, the compositions of Plitt suffer from poor heat stability which results in gradual discoloration of the adhesive film as well as the substrate to which the adhesive is applied. Davis indicates the aging discoloration is overcome by the addition of acids to the Plitt adhesive compositions.
As is known in the art, pressure sensitive adhesives comprise a class of adhesive compositions which may be formulated from a variety of materials such as gums and resins of both natural and synthetic derivation, which typically are not water-soluble. When coated on a suitable substrate, these adhesive compositions share the common characteristic of being aggressively and permanently tacky at room temperature and are thus able to adhere to surfaces by the application of nothing more than manual pressure. Adhesives of this type may be applied to various substrates such as paper, cloth and plastic films, and the resulting coated substrate may then be converted to tapes and labels which are especially useful in the packaging field for any number of applications involving marking, sealing, reinforcing or bonding.
In order to be satisfactory in commercial usage, a pressure sensitive adhesive must, of course, possess good tack and tack retention properties, that is, the adhesive must firmly adhere to various surfaces and the adhesive films thereof should retain their properties on aging. An equally important property of such an adhesive in many applications is that of high internal strength, i.e., good cohesion. High internal or cohesive strength is a necessary characteristic of any adhesive composition which is to be employed in preparing adhesive products which must support considerable amounts of weight. It is also necessary that the property of high internal strength be instilled in a pressure sensitive adhesive without adversely affecting its tack and tack retention properties. Previously, internal strength had been imparted to pressure sensitive adhesives by such means as incorporating high molecular weight polymers in the adhesive formulation and requiring an organic solvent, such as toluol or methanol. Methods such as these, however, have generally proved unsatisfactory since cohesive strength was attained only at the expense of sacrificing the tack and adhesion properties of the composition.
Polyvinyl methyl ether (PVME) has been taught as a major component in pressure sensitive adhesives. Kuramoto, et al. U.S. Pat. No. 3,657,396, teaches a polyvinyl methyl ether-based pressure sensitive adhesive but discloses that PVME exhibits adhesive properties but fails to exhibit cohesive properties and has a highly increased fluidity when used independently. Kuramoto improved the properties of the PVME adhesives by addition of acrylic polymers and copolymers having superior properties of adhesion and cohesion, as compared to those properties of PVME. The examples of Kuramoto indicate organic solvents, toluol and methanol, are required to formulate the improved adhesive compositions.
The general formula for a pressure sensitive adhesive includes an elastomeric polymer, a tackifying resin, any necessary filter, various antioxidants and stabilizers. The three physical properties which must be considered are shear strength, peel strength, and tack. As noted above, high molecular weight polymers are frequently incorporated into the adhesive formulation to improve cohesive strength. For example, U.S. Pat. No. 2,937,956 teaches a pressure sensitive adhesive formulation comprising a mixture as an emulsion or dispersion of 20 to 90% by weight of polyvinyl methyl ether and 10 to 80% by weight of a copolymer of acrylonitrile and a substance copolymerizable therewith selected from the group consisting of styrene, butadiene, vinylether, acrylic acid esters and methacrylic acid esters, and mixtures thereof.
Block copolymers have been used in the past to produce adhesive compositions with improved adhesive power or peel strength. Serious limitations have existed as to characteristics of these compounds which have required formula modifications to achieve desired properties in the final pressure sensitive adhesive composition. For example, U.S. Pat. No. 3,932,332 teaches copolymers of alpha-methylstyrene and styrene prepared under controlled conditions as necessary to impart tack to elastomeric block copolymers of styrene and isoprene such as Shell Chemical Company's Kraton.TM.D 1107. U.S. Pat No. 4,359,551 teaches that a specific tackifier mixture in a blend of a modified block copolymer can serve as a hot-melt pressure sensitive adhesive composition which is excellent in holding power especially for paper. U.S. Pat. No. 4,361,663 teaches incorporation of tackifier resins into pressure sensitive adhesive compositions of about 50 to about 150 parts by weight per 100 parts by weight of a block copolymer of polystyrene and a polydiene such as manufactured by Shell Chemical Company under the trade name Kration.TM.G.
Kraton.TM. block copolymers of polystyrene and a polydiene which have been conventionally used in adhesives technology have suffered from a serious drawback. The polydiene component in Kraton.TM. block copolymers can be highly unsaturated such as polybutadiene and polyisoprene. Because of this unsaturation, Kraton.TM. block copolymers having a high degree of unsaturation are highly susceptible to thermal and oxidative degradation. Shell Chemical Company, in order to overcome these undesirable properties, has developed a series of Kraton.TM. block copolymers in which the mid-unsaturated block is hydrogenated to yield a saturated mid-block, thus overcoming the problem of thermal and oxidative degradation. However, as taught in U.S. Pat. No. 4,361,663, it has been found that these saturated mid-block Kraton.TM. copolymers are difficult to tackify.
Kulzick, et al. U.S. Pat. No. 5,080,978, discloses a PVME-based adhesive composition for hot-melt application wherein the composition comprises, as a first component, at least about 10 weight percent of PVME, and, as a second component, at least 10 weight percent of a thermoplastic resin. The second component is selected from the group consisting of C.sub.2 to C.sub.6 polyolefins and copolymers such as amorphous polypropylene, ethylene vinyl acetate copolymer, low density polyethylene, polystyrene, and block copolymers thereof with isoprene and 1,3-butadiene and ethylenealkylacrylate copolymers. A tackifier resin can also be present. Kulzick '978 teaches the amount of thermoplastic resin in a two-component adhesive composition, or in a multiple component composition, is preferably more than 20 percent by weight of the total composition. Kraton.TM. 1107, made by Shell Chemical Company, Houston, Tex., Stereon 840A, made by Firestone Tire & Rubber Company, Akron, Ohio, and Vector 4113-D, made by Dexco Polymers, Houston, Tex., and hydrogenated variants, as well as ethylene-ethylacrylate copolymers, such as the DPD series made by Union Carbide, are taught as suitable thermoplastic resins. The adhesive compositions are prepared by blending the components in a melt at a temperature of about 160.degree. C. to about 200.degree. C. to obtain a homogeneous mixture and product that is essentially 100% solids. The tackifier component is optionable.
Pressure-sensitive adhesives are widely used in the form of adhesive tapes and labels. Conventional pressure sensitive adhesive tapes which provide good adhesive properties typically leave adhesive residues on the surface of the substrate when peeled. The adhesive residues are normally not removable unless an organic solvent is used to wipe the substrate. Since pressure-sensitive tapes and labels are frequently used to bond and/or identify industrial and consumer items, an adhesive residue can constitute a removal problem not only for the user but also for the marketer of the industrial or consumer product. The problem of removing the residue can therefore result in a marketing problem.
This invention relates to improved pressure-sensitive adhesive compositions which comprise a water-soluble component and a water dispersible component, and which, upon application and removal, leave a residue which is water-soluble and water-removable from the surface of the substrate without need of an organic solvent. The invented composition has high internal or cohesive strength, as measured by shear strength, and has good adhesion and tackiness. The adhesive properties of the composition compare favorably with conventional pressure sensitive adhesive compositions upon a variety of substrate materials, including metals and plastics. The pressure sensitive adhesive composition is especially suited for bonding polyethylene, polypropylene and ethylene-propylene copolymers. The invented composition can be formulated for use as a water emulsion, a solvent solution or as a hot-melt.
It has been found that by combining polyvinyl methyl ether with limited amounts of a water base dispersion of a block copolymer of polystyrene and a polydiene, such as made by Shell Chemical Company under the trade name of Kraton.TM., the resulting composition has superior adhesive properties. The water base dispersion of the block copolymer retains its water-dispersible properties as a dried component of an adhesive composition. Cohesive strength, peel strength and tack are equivalent to conventional pressure-sensitive adhesives. The adhesive residue, if any remains upon removal of the adhesive, is water-removable without application of an organic solvent. The pressure sensitive adhesive can be used as a hot-melt, a solvent solution or a water emulsion. The pressure sensitive adhesive composition is especially suited for bonding polyethylene, polypropylene, ethylene propylene copolymers, metal and paper.
The prior art has taught that pressure sensitive adhesive compositions containing PVME and a styrenic block copolymer can optionally require a tackifier resin, also that in a mixture of PVME and a block copolymer the block copolymer is preferably at least 20% by weight of the total two-component composition or about 25% of the weight of the PVME (U.S. Pat. No. 5,080,978). The prior art teaches that hydrogenated styrenic block co-polymer (Kraton.TM.) is difficult to tackify (U.S. Pat. No. 4,361,663), and that ratio of tackifying resin to Kraton.TM. needs to be about 50:100 to 150:100 parts by weight. The prior art teaches that a water-soluble pressure sensitive adhesive composition consists essentially of a mixture of (a) PVME and (b) a water-soluble acrylic polymer (U.S. Pat. No. 3,657,396).
However, the present invented composition comprising polyvinyl methyl ether and a water-dispersible styrenic block copolymer in a weight ratio of about 95:5 to about 88:12, parts by weight, is water removable, has good cohesive strength and good peel strength in the presence of less than about 14 weight percent or less of styrenic block copolymer and excellent tack, in the absence of a tackifying resin, irrespective of the hydrogenation of the styrenic block copolymer.